Method for processing elemental sulfur-bearing materials using high temperature pressure leaching

ABSTRACT

The present invention relates generally to a process for the production of sulfuric acid and liberation of precious metal values from materials containing sulfur through pressure leaching operations. In accordance with various aspects of the present invention, the sulfur-bearing materials may comprise residues from pressure leaching operations, such as those carried out at medium temperatures. The process of the present invention can be advantageously used to convert such sulfur-bearing materials to sulfuric acid by means of pressure leaching. The sulfuric acid so produced can be used beneficially in other mineral processing operations, for example those at the site where it is produced. Metals, such as precious metals, that are contained within the sulfur-bearing materials advantageously may be recovered from processing products by established precious metals recovery technology.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent Application,Serial No. 60/220,677 entitled “Methods for Conversion of Sulfur-bearingMaterial to Sulfuric Acid and Recovery of Associated Metals by HighTemperature Pressure Oxidation” filed on Jul. 25, 2000, which isincorporated by reference herein.

FIELD OF THE INVENTION

The present invention relates generally to a process for manufacturingsulfuric acid, and more specifically, to a process for manufacturingrelatively dilute sulfuric acid from sulfur-bearing materials using hightemperature pressure leaching processes and recovering metal values fromthe sulfur-bearing materials.

BACKGROUND OF THE INVENTION

Hydrometallurgical treatment of copper containing materials, such ascopper ores, concentrates, and the like, has been well established formany years. Currently, there exist many creative approaches to thehydrometallurgical treatment of these materials. The recovery of copperfrom copper sulfide concentrates using pressure leaching promises to beparticularly advantageous.

The mechanism by which pressure leaching releases copper from a sulfidemineral matrix, such as chalcopyrite, is generally dependent ontemperature, oxygen availability, and process chemistry. In hightemperature pressure leaching, typically thought of as being pressureleaching at temperatures above about 200° C., the dominant leachingreaction in dilute slurries may be written as follows:

4CuFeS₂+4H₂O+17O₂→4CuSO₄+2Fe₂O₃+4H₂SO₄  (1)

During pressure leaching of copper sulfide concentrates, such aschalcopyrite containing concentrates at medium temperatures (e.g., attemperatures in the range of between about 140° C. to about 180° C.),however, a significant fraction of the sulfide converts to elementalsulfur (S°) rather than sulfate (SO₄ ⁻²). According to the reaction:

4CuFeS₂+4H₂SO₄+5O₂→4CuSO₄+2Fe₂O₃+8S°+4H₂O  (2)

For example, experimental results show that at about 160° C. and about100 psi oxygen overpressure in the pressure leaching vessel, from about60 to about 70 percent of the sulfur in the super-finely ground coppersulfide concentrate is converted to elemental sulfur, with the remainderbeing converted to sulfate.

Elemental sulfur is a hydrophobic substance. In the pressure leachingprocess slurry, under certain temperature and solution conditions,sulfur has a tendency to agglomerate. Moreover, molten elemental sulfurbecomes highly viscous us at elevated temperatures. For example, theviscosity of molten sulfur increases from less than 100 centipoise at150° C. to more than 90,000 centipoise at 185° C. As such, the moltensulfur may tend to encapsulate metal values in the process slurry,including precious metals and unreacted metal sulfides, and/or stick tovarious parts of any apparatus in which processing operations on themolten sulfur are performed. Encapsulation of the metal values, forexample, copper, precious metals and the like, tends to make subsequentrecovery of such metal values extremely difficult using conventionalprocessing techniques. As discussed in applicant's co-pendingapplication entitled “Method for Recovery of Metals From MetalContaining Materials Using Medium Temperature Pressure Leaching” filedJul. 25, 2001 and assigned U.S. Ser. No. 09/915,105, the subject matterof which is hereby incorporated herein by reference, while pressureleaching under medium temperature conditions offers many advantages,prior medium temperature pressure leaching processes characteristicallyhave suffered from incomplete metal (e.g., copper) extraction resultingfrom either passivation of the metal sulfide particle surfaces or by themetal sulfide particles becoming coated with molten elemental sulfur. Asdiscussed in greater detail in applicant's co-pending application,proper control of such pressure leaching processes, as describedtherein, enables the formation of elemental sulfur in addition to thedesired metal recovery (e.g. copper). However, recovery of metal valuesthat may be contained in the elemental sulfur-containing residue, suchas, for example, precious metals, may be difficult with use ofconventional techniques, and as such they may be lost. Moreover, if theacid produced by such processing techniques could not be used at thesite where the recovery was performed, costs would be incurred inconnection with transportation of the residue or handling of the acid.An effective and efficient method to manufacture sulfuric acid fromsulfur-bearing material, particularly elemental sulfur-containingresidue resulting from pressure leaching operations operated at mediumtemperatures (e.g., about 140° C. to about 180° C.) is needed. Moreover,an effective and efficient method to enhance recovery of any metalvalues encapsulated within the sulfur-bearing material would beadvantageous.

SUMMARY OF THE INVENTION

While the way in which the present invention addresses the deficienciesand disadvantages of the prior art is described in greater detailhereinbelow, in general, according to various aspects of the presentinvention, a process for manufacturing sulfuric acid includes pressureleaching of sulfur-bearing materials, preferably at high temperatures,not only to facilitate the recovery of a sulfuric acid solution, butalso to enhance recovery of metal values contained in the sulfur-bearingmaterials. The acid produced, preferably a relatively dilute sulfuricacid solution advantageously can be used in other metal extractionprocesses, often with significant cost savings.

As will be described in greater detail hereinbelow, the methods andprocesses of the present invention are particularly suited for use inconnection with sulfur-bearing materials comprising residues frompressure leaching operations, such as, for example, those operated atmedium temperatures (e.g., about 140° to about 180° C.).

In accordance with an exemplary embodiment of the present invention, aprocess for manufacturing sulfuric acid from sulfur-bearing materialsgenerally includes the steps of: (i) providing a feed stream containinga sulfur-bearing material, and (ii) subjecting the sulfur-bearingmaterial feed stream to high temperature pressure leaching in a pressureleaching vessel, optionally in the presence of a suitable dispersingagent. In accordance with a preferred aspect of this embodiment of theinvention, the sulfur-bearing material feed stream comprises residuefrom medium temperature pressure leaching of a copper sulfide mineral,such as chalcopyrite or a blend of that residue combined with elementalsulfur. In accordance with a further preferred aspect of this embodimentof the invention, the use of a dispersing agent during pressure leachingmay aid in alleviating processing problems caused by the high viscosityand hydrophobic nature of elemental sulfur at higher temperatures (e.g.,above about 160° C.

In accordance with a further aspect of this embodiment of the presentinvention, metal values contained in the sulfur-bearing material feedstream are liberated from the elemental sulfur residue during pressureleaching, during which the elemental sulfur is converted to sulfuricacid, and then separated from the resultant acid stream and subjected tometal recovery processing. Such metal recovery processing may includeprecious metal recovery.

The present inventors have advanced the art of copper hydrometallurgy byrecognizing the advantages of not only producing a sulfuric acidsolution from sulfur-bearing materials, such as the elemental sulfurby-product of medium temperature pressure leaching of copper sulfideminerals, but also of enabling the recovery of metal values (e.g.,precious metals) entrained therein, which otherwise may have been lost.

These and other advantages of a process according to various aspects ofthe present invention will be apparent to those skilled in the art uponreading and understanding the following detailed description withreference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWING

The subject matter of the present invention is particularly pointed outand distinctly claimed in the concluding portion of the specification. Amore complete understanding of the present invention, however, may bestbe obtained by referring to the detailed description and claims whenconsidered in connection with the drawing figures, wherein like numeralsdenote like elements and wherein:

FIG. 1 illustrates a flow diagram of a process in accordance with anexemplary embodiment of the present invention;

FIG. 2 illustrates a flow diagram of further processing in accordancewith the embodiment of the present invention illustrated in FIG. 1; and,

FIG. 3 illustrates a graphical profile of sulfuric acid yield versustemperature in accordance with various embodiments of the presentinvention.

DETAILED DESCRIPTION OF AN EXEMPLARY EMBODIMENT OF THE INVENTION

The present invention exhibits significant advancements over prior artprocesses, particularly with regard to process efficiency and processeconomics. Moreover, existing metal (e.g., copper) recovery processesthat utilize conventional atmospheric or pressure leaching/solventextraction/electrowinning process sequences may, in many instances, beeasily retrofitted to exploit the many commercial benefits the presentinvention provides.

Referring now to FIG. 1, in accordance with various aspects of oneembodiment of the present invention, a sulfuric acid production processpreferably involves providing a sufficient supply of a sulfur-bearingmaterial 102. In the context of the present invention, the term“sulfur-bearing material” refers to elemental sulfur, elementalsulfur-bearing material generated as a by-product of other metalrecovery processes, materials containing iron sulfides, copper sulfidesand/or other metal sulfides, or any combination of these. In addition,the term “sulfur-bearing material” refers to other sulfur compositionsthat may include sulfur together with any other sulfides and/or metalsthat might be attendant to or part of such sulfur compositions. Forpurposes of this disclosure, in most instances, the term “elementalsulfur,” for example as that term is used in FIG. 1, is usedinterchangeably with the term “sulfur-bearing material,” inasmuch as, aswill be clear from the following disclosure, the elemental sulfur andsulfide sulfur components of any sulfur-bearing material 102 areadvantageously converted to sulfuric acid in accordance with the presentinvention.

In accordance with one aspect of a preferred embodiment of the presentinvention, sulfur-bearing material feed stream 102 preferably comprisesthe sulfur-containing residue produced in connection with the pressureleaching of copper-containing material feed streams. As explained ingreater detail in Applicant's co-pending application, U.S. Ser. No.09/915,105, such copper-containing materials include copper sulfideores, such as, for example, ores and/or concentrates containingchalcopyrite (CuFeS₂) or mixtures of chalcopyrite with one or more ofchalcocite (Cu₂S), bornite (Cu₅FeS₄), and covellite (CuS). Thesulfur-containing residues that result from the pressure leaching ofsuch copper-containing material feed streams may advantageously beprocessed in accordance with the various aspects of the presentinvention.

Sulfur-bearing material feed stream 102 may be prepared for processingin any suitable manner. For example, desired composition and/orcomponent parameters can be achieved through a variety of chemicaland/or physical processing stages, the choice of which will depend uponthe operating parameters of the chosen processing scheme, equipment costand material specifications. For example, feed stream 102 may undergocomminution, blending, and/or slurry formation, as well as chemicaland/or physical conditioning. Such preparation efforts may include, forexample, sulfur-bearing material feed stream 102 being combined withsolution, for example, pregnant leach solution (PLS) or barren raffinatesolution from an existing acid heap leaching operation or an agitatedtank leaching operation, in a repulp process to produce a slurry.

With continued referenced to FIG. 1, preferably sulfur-bearing materialfeed stream 102 (or slurry) is suitably combined with a fluid 14,preferably water, and suitable amounts of an oxygenating supply, forexample, oxygen 12, optionally with one or more dispersing agents 16 tofacilitate pressure leaching (step 104) of sulfur-bearing material feedstream 102. In accordance with one aspect of the present invention, thefeed slurry containing sulfur-bearing material 102 may be formed in anysuitable mixing vessel or by in-line blending. Other additives, such aswetting agents or the like, for example, lignosulfonates, may also beused.

As those skilled in the art will understand, elemental sulfur isoptimally oxidized to sulfuric acid according to the following reaction:

2S°+3O₂+2H₂O→2H₂SO₄

Further, as those skilled in the art will appreciate, this reaction mayproceed more completely as temperature is increased. In addition, wherethe sulfur-bearing material feed 102 comprises hematite and/or otheriron-bearing materials, basic iron sulfate may be formed during pressureleaching according to the following reaction:

Fe₂O₃+2SO₄ ²⁻+4H⁺→2Fe(OH)SO₄+H₂O

When basic iron sulfate is formed, acid is consumed and subsequent metalrecovery may be inhibited. As such, to enable efficient acid productionand to optimize metal recovery, the pulp density of the feed provided tothe pressure leaching vessel should be controlled.

In accordance with various aspects of the present invention, suitableamounts of water 14 and oxygen 12 are advantageously provided to feedstream 102 to facilitate the reaction of elemental sulfur and sulfidesulfur to sulfuric acid. Further, the feed slurry (i.e., sulfur-bearingmaterial 102) provided for pressure leaching, in accordance with variousaspects of the present invention, preferably contains sulfur and othermaterials, including, without limitation, metal values such as copper,molybdenum, precious metals and the like.

Sulfur-bearing material feed 102 provided to pressure leaching vessel104 preferably has a percent solids ranging from about 2 to about 20percent, more preferably on the order of about 3 to about 8 percentsolids. In some cases, feed 102 may preferably be combined withadditional elemental sulfur, such as from an external source, and insuch cases higher percent solids may be tolerated. Where feed 102includes a significant amount of iron, then the acid concentration ofthe material in pressure leaching vessel 104 is advantageouslycontrolled to from about 20 to about 50 grams per liter, and morepreferably in the range of about 30 to about 40 grams per liter acid.

With continued reference to FIG. 1, after sulfur-bearing material feedstream 102 has been suitably prepared, it is subjected to processing,preferably pressure leaching processing, and more preferably hightemperature pressure leaching. As used herein, the term “pressureleaching” refers to a process in which the sulfur-bearing material iscontacted with oxygen under conditions of elevated temperature andpressure. During pressure leaching, the elemental sulfur of thesulfur-bearing material 102 and many of the metal sulfides contained infeed 102 are oxidized to form sulfate and dissolved metal ions insolution. In some cases, significant metal values may remain in thesolid residue including precious metals, molybdenum and others.

The pressure leaching processes suitably employed in connection with thepresent invention are generally dependent upon, among other things,temperature, oxygen availability, and process chemistry. While variousparameters of each may be utilized, in accordance with preferred aspectsof the present invention, the temperature during pressure leachingpreferably is maintained above about 220° C., and more preferably in therange of about 235° C. to about 275° C., and optimally in the range ofabout 250° C.

The duration of pressure leaching in any particular application dependsupon a number of factors, including, for example, the characteristics ofthe feed material (e.g., sulfur-bearing material feed stream 102) andthe pressure leaching process pressure and temperature. Preferably, theduration of pressure leaching in accordance with various aspects of thepresent invention ranges from about 0.5 to about 3 or more hours, andoptimally is on the order of about one hour.

While any reactor vessel for pressure leaching may be used, preferablyan agitated, multiple-compartment pressure leaching vessel is employed.For example, any pressure containment or pressure controlled system maybe used. Agitation may be accomplished in any conventional manner, andpreferably is sufficient to suitably disperse sulfur-bearing materialfeed stream 102, as well as any other additives within the pressureleaching vessel.

The present inventors have found that to prevent the formation of sulfuragglomerates, the temperature in the pressure-leaching vessel preferablyshould be maintained above about 220° C., and more preferably aboveabout 235° C. and most preferably about 250° C. Moreover, the presentinventors have found that the optional addition of certain dispersantsand/or particulate matter, for example, ground sand and the like,facilitates enhanced sulfuric acid recovery as well as enhanced metalvalue recovery, especially precious metal recovery.

With momentary reference to FIG. 3, the difficulties occasioned bysulfur can be addressed through use of elevated temperature, for examplethrough the use of elevated temperatures in the range of about 250° C.and/or with the use of various dispersants. For example, as shown, theuse of ground sand as a dispersant tends to enhance acid yield. As such,in accordance with an optional aspect of the present invention, adispersing agent is added to sulfur-bearing material feed stream 102either during formation of the feed slurry or to the pressure leachingvessel used in pressure leaching step 104. Suitable dispersants includeany substantially inert particle, such as ground sand or mineralprocessing tailings, or other particles that tend to provide for theadherence of sulfur and increase the exposed surface area of the sulfurto be oxidized. Other suitable dispersants may include recycled pressureleaching residue, precious metal recovery residues (e.g., cyanidationtailings) or the like. In general, any material now known or hereafterdevised by those skilled in the art which advantageously serve suchpurposes may be used.

During pressure leaching 104, oxygen is added to the pressure leachingvessel, preferably substantially continuously, to maintain the oxygenoverpressure at optimal levels for the desired chemical reactions toproceed. That is, sufficient oxygen is suitably injected to maintain anoxygen partial pressure in the pressure leaching vessel ranging fromabout 50 to about 150 psig. The total pressure in the sealed pressureleaching vessel is preferably from about 600 to about 800 psig.

In any event, in accordance with various aspects of the presentinvention, a product a slurry is preferably obtained from pressureleaching processing 104 in a conventional manner. Prior to subsequentprocessing, the resultant product slurry is preferably caused to achieveapproximately ambient conditions of pressure and temperature. Forexample, the product slurry may be flashed to release pressure and toevaporatively cool the slurry through the release of steam.

However, the temperature and pressure of the product slurry may beadvantageously reduced in any manner now known or hereafter devised.

In accordance with various preferred aspects of the present invention,once the temperature and pressure of the product slurry is appropriatelyreduced, preferably, one or more solid-liquid phase separation stages(step 106) may be used to separate the sulfuric acid solution from thesolid particles in the product slurry. This may be accomplished in anyconventional manner, including use of filtration systems,counter-current decantation (CCD) circuits, thickeners, and the like. Avariety of factors, such as the process material balance, environmentalregulations, residue composition, economic considerations, and the like,may affect the decision whether to employ a CCD circuit, a thickener, afilter, or any other suitable device in a solid-liquid separation stage.However, it should be appreciated that any technique for conditioningthe product slurry is within the scope of the present invention. Theproduct slurry is subjected to solid-liquid phase separation (step 106)to yield a resultant liquid phase sulfuric acid solution 108 and a solidphase residue 18.

Preferably, solid-liquid phase separation (step 106) is accomplishedthrough the use of multiple stages of counter current decantation (CCD)washing. Wash solution and a suitable flocculant may be added asdesired.

Sulfuric acid solution 108 may be used in a number of ways. For example,all or a portion of solution 108 may be used in other processingoperations. The production of sulfuric acid in this manner mayadvantageously reduce costs typically associated with acid procurementfor such processing operations. Such processing operations may include,among other things, acid-consuming heap leaching operations used inconnection with pressure leaching operations or otherwise, agitated tankleaching, combinations thereof or other processing operations.

On the other hand, the solid residue 18 obtained from solid-liquid phaseseparation (step 106) may be further processed. For example, withcontinued reference to FIG. 1, if the metal content of the washed solidsfrom solid-liquid separation step 106 is sufficiently high to warrantfurther processing, the metals contained therein may be recoveredthrough conventional means such as, for example, through smelting orestablished metal recovery processing (e.g., precious metal recovery), apreferred process for which will be described in greater detailhereinbelow in connection with FIG. 2. If, however, the metals contentof residue 18 is too low to justify further treatment, the residue maybe sent to an impoundment area (not shown).

Referring now to FIG. 2, residue 18 from liquid-solid phase separationstep 106 (FIG. 1) may be subjected to various further processing torecover metals contained therein, particularly precious metals, such asgold and silver, which may exist in the residue. Depending on thecharacteristics of residue 18, it may be advantageous to subject it toneutralization and/or pH adjustment, such as is illustrated in step 202.The residue once so treated may then be subjected to further processingor otherwise utilized. Such processing may include, with continuedreference to FIG. 2, an optional hot lime boil (step 204) followed byprecious metal recovery (step 208), such as through the use ofconventional cyanide leaching (step 206) followed by liquid-solid phaseseparation (step 210). If cyanide leaching is used, the resultanttailings may be recycled and utilized elsewhere in connection with ahydrometallurgical process, for example as a sulfur dispersant, (notshown), Typically after the cyanide is destroyed (step 212).Alternatively, the tailings may be disposed (step 214). As those skilledin the art will recognize, any number of precious metal or other metalrecovery methods may be suitable to achieve the objective of recoveringmetals, such as precious metals (e.g., as silver and gold) from residuestream 18, and therefore alternative processing routes may besuccessfully utilized.

The Examples set forth hereinbelow are illustrative of various aspectsof certain preferred embodiments of the present invention. The processconditions and parameters reflected therein are intended to exemplifyvarious aspects of the invention, and are not intended to limit thescope of the claimed invention.

EXAMPLE 1

Various sulfur pressure leaching tests were performed. A Parr batch 2.0liter pressure leaching vessel was utilized. In each instance, elementalsulfur was combined in the pressure leaching vessel with oxygen andwater to form a slurry, and the slurry was contained in a non-adhesiveliner. The reaction temperature was varied as shown in Table 1. In eachinstance, the reaction was permitted to operate for one hour. Fiftygrams of sulfur with 100 psi oxygen overpressure were provided.

Yields were obtained by observing the amount of acid produced ascompared to the amount of elemental sulfur provided (a theoretical yieldof 100% was calculated to represent 3.06 grams H₂ SO₄/g sulfur).

As can be seen from the results shown in Table 1, enhanced acid yieldswere obtainable with enhanced temperature and the utilization of adispersant, such as ground sand, mineral processing tailings, or othersuitable material.

TABLE 1 O₂ Usage gO₂/ % of H₂SO₄ Temp. Time % g reacted theoreticalStrength Yield Test (° C.) (min.) % S° Sand S° 1.5 g/g S° (g/L) g/g S° %A 160 65 5 15  5.08 339 1.6   0.02 0.7  B 220 60 5 0 1.88 126 69  1.5550.8 C 220 60 5 5 1.78 n/a 84  1.75 57.1 D 235 55 5 0 1.88 126 114 2.3877.4 E 235 60 5 5 1.82 122 121 2.63 86.0 F 250 60 5 0 1.92 128 129 2.7088.4 G 250 60 5 5 2.08 139 134 2.83 92.4

EXAMPLE 2

A medium temperature pressure leaching residue containing 23.8 wt %elemental sulfur was prepared for pressure leaching by making a feedslurry having 10.4 wt % solids with synthetic raffinate and water. Thefeed was provided to a stirred 2.0 liter Parr pressure leaching vesselat 225° C. with 50 psi oxygen overpressure for 60 minutes. The resultingsolution contained 55.9 g/L free acid and a bulk residue (containing2.9% elemental sulfur and 5.1% sulfate). Precious metals were recoveredfrom the residue in acceptable quantities (i.e., 88% gold and 99% silverextraction).

The graphical profile of FIG. 3 further illustrates the benefits onsulfuric acid yield as a function of temperature and dispersant additionin accordance with various embodiments of the present invention. Theseresults generally indicate that sulfuric acid production increases withincreasing temperature. Moreover, the comparison of Curve 32 versusCurve 34 illustrates sulfuric acid yield can be enhanced, on the orderof between about 5 and about 10%, with the addition of a suitabledispersant, for example, ground sand.

An effective and efficient method of producing sulfuric acid from anelemental sulfur-bearing material has been presented herein. The use ofa dispersing agent as well as elevated temperatures during pressureleaching may aid in alleviating processing problems caused by the highviscosity of elemental sulfur. Further, the present inventors haveadvanced the art of copper hydrometallurgy by recognizing the advantagesof not only producing sulfuric acid solution from sulfur-bearingmaterials, such as by-products of medium temperature pressure leachingof copper sulfide minerals, but also enabling the recovery of metals,such as precious metals, entrained therein, which otherwise may havebeen lost.

The present invention has been described above with reference to anumber of exemplary embodiments and examples. It should be appreciatedthat the particular embodiments shown and described herein areillustrative of the invention and its best mode and are not intended tolimit in any way the scope of the invention as set forth in the claims.Those skilled in the art having read this disclosure will recognize thatchanges and modifications may be made to the exemplary embodimentswithout departing from the scope of the present invention. These andother changes or modifications are intended to be included within thescope of the present invention, as expressed in the following claims.

What is claimed is:
 1. A treatment process comprising the steps of: a)providing an elemental sulfur-bearing material; b) pressure leachingsaid sulfur-bearing material at a temperature in the range of about 220°C. to about 275° C. in an oxygen-containing atmosphere in an agitatedmultiple-compartment pressure leaching vessel to form a product slurrycomprising a sulfuric acid solution; c) separating at least a portion ofsaid sulfuric acid solution from said product slurry to yield a residue;d) recovering at least one metal value from said residue; e) adding asufficient amount of ground sand or mineral processing tailings to saidpressure leaching vessel during pressure leaching.
 2. The process ofclaim 1, wherein said elemental sulfur bearing material comprises asulfur product stream from a pressure leaching operation carried out ata temperature in the range of about 140° C. to about 180° C.
 3. Theprocess of claim 1 wherein said step of recovering metal values fromsaid residue comprises recovering at least one precious metal from saidresidue.
 4. The process of claim 1 wherein said step of pressureleaching said sulfur-bearing material comprises pressure leaching attemperature above about 235° C.
 5. The process of claim 1 wherein saidstep of pressure leaching said sulfur-bearing material comprisespressure leaching at temperatures in the range of about 250° C.
 6. Aprocess for recovering metal values from the solid residue of a pressureleaching process carried out at a temperature in the range of about 140°C. to about 180° C., the process comprising the steps of: a) comminutingthe solid residue from the pressure leaching process carried out at atemperature in the range of about 140° C. to about 180° C. to produce afeed material; b) forming a feed slurry by combining said feed materialwith a sufficient amount of fluid medium and by adding a sufficientamount of ground sand or mineral processing tailings; c) pressureleaching said feed slurry at a temperature in the range of about 220° C.to about 275° C. in an oxygen-containing atmosphere to yield a pressureleach product slurry comprising a sulfuric acid solution; d) reducingthe temperature and pressure of said product slurry; e) separating atleast a portion of said sulfuric acid solution from said product slurryto yield a solid residue; f) recovering at least one metal value fromsaid solid residue.
 7. The process of claim 6 wherein said step ofrecovering at least one metal value from said solid residue comprisesrecovering one or more precious metals contained in said residue.
 8. Theprocess of claim 6 wherein said step of reducing the temperature andpressure of said product slurry comprises flashing said product slurry.9. The process of claim 6, said process further comprising the step ofutilizing at least a portion of said sulfuric acid solution inconnection with other processing operations.
 10. The process of claim 6wherein said step of pressure leaching said feed slurry is conducted ata temperature in the range in excess of about 235° C.
 11. A process forrecovering metal values from the solid residue of a pressure leachingprocess carried out at a temperature in the range of about 140° C. toabout 180° C., the process comprising the steps of: a) comminuting thesolid residue from the pressure leaching process carried out at atemperature in the range of about 140° C. to about 180° C. to produce afeed material; b) forming a feed slurry by combining said feed-materialwith a sufficient amount of fluid medium; c) pressure leaching said feedslurry at a temperature in the range of about 220° C. to about 275° C.in an oxygen-containing atmosphere to yield a pressure leach productslurry comprising a sulfuric acid solution; d) adding a sufficientamount of ground sand or mineral processing tailings during saidpressure leaching step; e) reducing the temperature and pressure of saidproduct slurry, f) separating at least a portion of said sulfuric acidsolution from said product slurry to yield a solid residue; g)recovering at least one metal value from said solid residue.
 12. Theprocess of claim 11 wherein said step of recovering at least one metalvalue from said solid residue comprises recovering one or more preciousmetals contained in said residue.
 13. The process of claim 11 whereinsaid step of reducing the temperature and pressure of said productslurry comprises flashing said product slurry.
 14. The process of claim11, said process further comprising the step of utilizing at least aportion of said sulfuric acid solution in connection with otherprocessing operations.
 15. The process of claim 11 wherein said step ofpressure leaching said feed slurry is conducted at a temperature in therange in excess of about 235° C.
 16. A process for the production ofsulfuric acid and recovery of precious metals from an elementalsulfur-bearing material comprising the steps of: a) providing anelemental sulfur-bearing material; b) pressure leaching said elementalsulfur-bearing material at a temperature in the range of about 220° C.to about 275° C. in an oxygen-containing atmosphere in an, agitatedmultiple-compartment pressure leaching vessel to form a product slurrycomprising a sulfuric acid solution; c) adding a dispersant comprising asufficient amount of ground sand or mineral processing tailings duringsaid pressure leaching step; d) separating at least a portion of saidsulfuric acid solution from said product slurry to yield a solidresidue; e) recovering at least one precious metal value from said solidresidue.
 17. The process of claim 16 wherein said step of providing anelemental sulfur-bearing material comprises providing asulfur-containing product stream from a pressure leaching operationcarried out at a temperature in the range of about 140° C. to about 180°C.
 18. The process of claim 16 wherein said step of pressure leachingcomprises pressure leaching at a temperature of about 235° C.